The thermodynamics of ionization of gaseous oxides; the first ionization potentials of the lanthanide metals and monoxides

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Abstract
The first ionization potentials of the gaseous lanthanide metals and monoxides have been determined by electron impact from the appearance potentials of ionization efficiency curves. A method of simultaneous and intercomparative measurements with known standards was used and the results for the lanthanide metals are in excellent agreement with values obtained previously from spectroscopic and surface ionization studies. In the early part of the lanthanide sequence the ionization potentials of LnO(g) are less than those of Ln(g), whereas the converse is true in the latter part. The differences in the ionization potentials of LnO(g) and Ln(g) are simply related to the differences in the dissociation energies of LnO(g) and LnO+(g). Values of D0(LnO+) are derived. The nature of the chemical bonding in LnO(g) and LnO+(g) is examined for the lanthanide sequence by means of an electrostatic point‐charge model. The assumption of monotonic variation of the interatomic distance and the electrostatic repulsion parameter and the nonmonotonic variation of the polarization energy derived from the known f‐to‐d electronic transitions are all mutually consistent and describe rather closely the variation in bonding in LnO(g); a similar scheme without the f‐to‐d transition energies is shown to describe the variation in the bonding of LnO+(g).